The present invention relates to a cutting machine for cutting a workpiece and, particularly, for cutting a semiconductor wafer with a required depth of cutting.
In the production of semiconductor chips as is well known among people skilled in the art, a number of rectangular regions are sectioned on the surface of a semiconductor wafer by streets that are arranged in a lattice form, and a semiconductor circuit is arranged in each of the rectangular regions. The semiconductor wafer is separated along the streets into individual rectangular regions to obtain semiconductor chips. A cutting machine used for separating the semiconductor wafer along the streets includes a chucking means for holding the semiconductor wafer that is to be cut, a cutting means for cutting the semiconductor wafer held on the chucking means, a first moving means for moving the chucking means relative to the cutting means in a moving direction perpendicular to the center axis of the chucking means, and a second moving means for moving the cutting means relative to the chucking means in a direction of depth of cutting, which is the direction of center axis of the chucking means. In this cutting machine, a position of the cutting means in the direction of depth of cutting relative to the chucking means is set by the second moving means, and the chucking means is moved relative to the cutting means by the first moving means to cut the semiconductor wafer. The depth of cutting the semiconductor wafer is defined by the position of the cutting means in the direction of cutting relative to the chucking means and by the thickness of the semiconductor wafer held on the chuck.
Generally, the thickness of the semiconductor wafer is nearly the same in each lot and hence, only one piece of semiconductor wafer in each lot is measured for its thickness manually to recognize the thickness of the semiconductor wafer to be cut. In the cutting of the semiconductor wafer by the cutting machine, the depth of cutting the semiconductor wafer is set by setting the position of the cutting means in the direction of cutting relative to the chucking means at a predetermined position, without detecting the thickness of the individual semiconductor wafers that are to be cut. However, the individual semiconductor wafers in each lots fluctuate a bit in thickness and hence, the depth of cutting the semiconductor wafers varies also in the individual semiconductor wafers, even though it is a slim amount.
Meanwhile, it is important to set the depth of cutting the semiconductor wafer very precisely and it is desired to avoid a change in the depth of cutting the semiconductor wafers that is ascribed to a change in the thickness of the individual semiconductor wafers. For example, it has been done to form a groove of a V-shape in cross section along the streets in the surface of the semiconductor to chamfer the four side edges of the surfaces of the individual rectangular regions prior to completely cutting the semiconductor wafer along the streets to separate it into the individual rectangular regions. Even in forming the V-shaped grooves, however, it is important to sufficiently precisely set the depth of cutting to sufficiently precisely set the size of chamfering. In recent years, further, a mode of separation into rectangular regions or a so-called predicing, has been put into practice. In this mode, too, it is important very precisely set the depth of cutting the semiconductor wafer. In the mode of separation into rectangular regions called dicing-before-grinding, the semiconductor wafer is cut to form grooves of a predetermined depth from the surface along the streets arranged in a lattice form. Then, the back surface of the semiconductor wafer is ground to decrease the thickness of the semiconductor wafer up to a thickness equal to the depth of the grooves thereby to separate the semiconductor wafer into the individual rectangular regions.
Japanese Laid-open Patent Publication (Kokai) No. 261907/1987 (JP-A 62-261907) discloses an art in which a gap between a microscope system and the surface of chucking means and a gap between the microscope system and the surface of a semiconductor wafer held on the chucking means are measured by an automatic focusing technology using the microscope system disposed above the chucking means that is located at a predetermined position, and the thickness of the semiconductor wafer is calculated from the measured values. According to this prior art, further, the position of the cutting means in the direction of cutting is set relative to the chucking means based on the calculated thickness of the semiconductor wafer, thereby to set the depth of cutting the semiconductor wafer. When a transparent or semitransparent film such as glassy film is formed on the surface of the semiconductor wafer, however, it is not possible to detect the thickness of the semiconductor wafer precisely by the above automatic focusing technology, which is a serious problem.
It is therefore a principal object of the present invention to provide a-cutting machine particularly suited for cutting a semiconductor wafer, which is capable of detecting the thickness of a workpiece held on the chucking means with a sufficient degree of precision even when a transparent or semitransparent film exists on the surface of the workpiece, and which can set the depth of cutting the workpiece with a sufficient degree of precision based on the detected thickness of the workpiece, without the need of bringing a probe into contact with the surface of the workpiece such as the semiconductor wafer and hence, without possibility of damaging the surface of the workpiece.
In order to accomplish the above-mentioned principal object according to the present invention, there is provided a cutting machine comprising a chucking means for holding a semiconductor wafer that is to be cut; a cutting means for cutting the semiconductor wafer held on the chucking means; a first moving means for moving the chucking means relative to the cutting means in a moving direction perpendicular to the center axis of the chucking means; a second moving means for moving the cutting means relative to the chucking means in a direction of depth of cutting, which is the direction of center axis of the chucking means; a thickness detecting means for detecting the thickness of the workpiece held on the chucking means; and a control means for controlling the motion of the second moving means depending upon the thickness of the workpiece detected by the thickness detecting means and for setting the position of the cutting means in the direction of depth of cutting relative to the chucking means thereby to set the depth of cutting the workpiece by the cutting means;
wherein the thickness detecting means includes a non-contact back-pressure sensor having a nozzle for flowing out a gas toward the surface of the workpiece held on the chucking means.
Preferably, the nozzle of the non-contact back-pressure sensor is mounted to freely move in the direction of depth of cutting. In a preferred embodiment, the chucking means has an adsorbing surface for vacuum-adsorbing the workpiece; and the thickness detecting means includes a nozzle position detecting means for detecting a gap from an original position of the nozzle where an end of the nozzle comes in contact with the adsorbing surface of the chucking means to the present position of the nozzle, and a thickness calculation means for calculating the thickness of the workpiece from the gap between the end of the nozzle and the surface of the workpiece on the chucking means detected by the non-contact back-pressure sensor and from the gap between the original position of the nozzle and the present position of the nozzle detected by the nozzle position detecting means. Further, the workpiece is a semiconductor wafer.